The present invention relates generally to electrochemical machining, and, more specifically, to electrochemical machining of blisks used in gas turbine engines.
In a gas turbine engine, air is pressurized in a multistage axial compressor and mixed with fuel in a combustor for generating hot combustion gases which are discharged through several rows of turbine stages A high pressure turbine powers the compressor, and a low pressure turbine powers a fan in a turbofan configuration for powering an aircraft in flight.
In the fan and compressor, rotor blades extend radially outwardly from a supporting disk for pressurizing the air during operation. The rotor blades commonly include dovetails at their radially inner ends which are mounted in complementary dovetail slots in the perimeter of the supporting disk. In this configuration, the disk and individual blades may be separately manufactured and later assembled together in the engine.
In another configuration, the rotor blades are integrally formed with their supporting disk in a one-piece or unitary assembly typically referred to as a blisk. The blades do not include dovetails, and extend radially outwardly from the supporting disk in a unitary assembly.
The blisk construction enjoys performance advantages in the engine, yet requires special manufacture thereof. Since the blisk includes a full row or complement of rotor blades, damage to any one of the many blades during the manufacturing process must be carefully avoided since the manufacture of even one unacceptable blade renders the entire blisk unacceptable leading to the scrapping thereof at considerable expense in material and manufacturing cost.
Fan blades are a special type of compressor blades and are larger in size for pressurizing a large volume of air for producing propulsion thrust, which bypasses the compressor during operation. The various forms of compressor blades have generally concave pressure sides and generally convex suction sides extending in span from root to tip of the blades and axially between leading and trailing edges.
The blades in a blisk have camber, twist, and suitable solidity selected for maximizing aerodynamic performance thereof. However, high camber, high twist, and high solidity create substantial problems for the manufacture of the blisk either by conventional machining or by electrochemical machining (ECM).
U.S. Pat. No. 4,851,090, assigned to the present assignee, discloses and claims a method and apparatus for electrochemically machining blisk blades. A pair of electrode tools conforming to the desired configurations of the pressure and suction sides of the individual blades are both translated and collectively rotated during ECM as a liquid electrolyte flows between the blade and tools. The blade forms an anode and the tools form cathodes provided with high electrical current for electrochemically machining the blade to the desired final dimensions thereof by surface erosion of the metal.
The blisk is mounted on a spindle which is rotated during operation to index individual blades between the tool pair, with the individual blade also being translated with the spindle for moving inwardly between the electrode tools. In this way, the compound movement of the electrode tools and the blade are used for electrochemical machining the individual blades in sequence for achieving the desired aerodynamic contours thereof, including camber and twist for the full row of blades on the supporting disk.
However, the ECM machine necessarily requires suitable setup. The blisk requires a fixture for mounting it to the machine spindle. The electrode tools must be correspondingly mounted to the supporting rotary head of the machine for independent translation thereof and collective rotation.
The machine is computer controlled using a conventionally developed computer numerical control (CNC) program which is loaded into the machine memory for use in machining the blisk blades in sequence.
In view of the precision requirements for the final blisk dimensions down to about a few mils or even less than one mil, an elaborate setup procedure is required to ensure precise machining of the production blisk. Either the production blisk itself, or a scrap blisk may be used as an initial sample loaded into the machine for machining one or more sample blades thereon. The sample must then be removed from the machine and inspected to accurately determine the dimensions thereof, which are then compared with the desired final dimensions for the blades.
The setup procedure is typically effected with incremental machining of the sample blades to avoid excessive machining thereof which would render the blade out-of-specification, and therefore unacceptable. Accordingly, the setup procedure is normally repeated several times to incrementally machine the sample blades, and correspondingly adjust tool mounting, blisk fixturing, or datum offsets for the CNC program as required to ensure proper alignment of the blisk in the machine, proper alignment of the electrode tools on their supporting head, and proper machining of the individual blisk blades.
When the setup procedure is finally completed, the production blisk may then be mounted in the machine in the same manner as the sample blisk, and without changing the mounting of the electrode tools or alignment of the various components of the machine. The production blisk may then be electrochemically machined blade-by-blade in sequence to the precise tolerances required by the corresponding drawing specifications therefor.
The manufacture of gas turbine engine blisks is made even more complex for tandem blisks. A tandem blisk includes two rows or stages of rotor blades extending radially outwardly from corresponding supporting disks, all of which are integrally joined together in a unitary or one-piece part. The two stages have correspondingly different configurations for the required aerodynamic performance thereof. Accordingly, different electrode tools are required for the different blisk stages, and corresponding setup of the ECM machine is required for machining each of the two stages of the tandem blisk.
The same machine described above has been used to manufacture tandem blisks in this country for many years. The electrode tools are substantially identical to each other for the two stages of the tandem blisk except for the required differences in the cutting surfaces thereof for effecting the different configurations of the two stages.
The machine is set up with one pair of tools for one stage followed by the final machining of that stage. The machine is then re-set up with a second set of electrode tools for the second stage followed by final machining thereof. And, suitable means are used to translate the spindle to align the different stages with the common rotary head. In this way, the same machine may be used in two independent and separate operations for machining the two stages of the tandem blisk.
Since the corresponding setup required for each of the two stages is an elaborate process, the corresponding manufacturing time and costs are correspondingly higher.
Accordingly, it is desired to provide an improved method of electrochemically machining a tandem blisk for reducing manufacturing time and costs.